What does SetHandler "proxy:unix:..." do?
Solution 1:
I finally found the answer in the docs for ProxyPass:
In 2.4.7 and later, support for using a Unix Domain Socket is available by using a target which prepends unix:/path/lis.sock|. For example, to proxy HTTP and target the UDS at /home/www.socket, you would use
unix:/home/www.socket|http://localhost/whatever/.